Air conditioning system for a motor vehicle

ABSTRACT

An air conditioning system ( 10 ) for a passenger compartment of a motor vehicle comprises a first heat exchanger ( 14 ) positionable outside the passenger compartment; a second heat exchanger ( 18 ) positionable inside the passenger compartment; an expansion device ( 16 ) positioned in a first fluid passage ( 22 ) between the heat exchangers; an electronically controlled variable stroke compressor ( 12 ) for pumping fluid into the second fluid passage and either in a first direction (X) or in a second direction (Y); reverse flow means ( 32 ) in the second fluid passage for controlling the direction of the flow of fluid; first sensing means ( 36 ) providing a first output signal indicative of the actual temperature of the air leaving the second heat exchanger; second sensing means ( 40 ) providing a second output signal indicative of the humidity of the air leaving the second heat exchanger; third sensing means ( 46 ) providing a third output signal indicative of the temperature of the air in the passenger compartment; and control means ( 26,30 ) electrically connected to the first, second and third sensing means and to the compressor for receiving the first, second, and third output signals, for determining a maximum temperature for the air leaving the second heat exchanger dependent on the second and third output signals, for comparing the determined maximum temperature to the actual temperature, and for controlling the stroke of the compressor to maintain the actual temperature at or below the determined maximum temperature during fluid flow in the second direction. Reduces the risk of unacceptable levels of hot saturated air reaching the passenger compartment.

TECHNICAL FIELD

The present invention relates to an air conditioning system for thepassenger compartment of a motor vehicle, and more particularly to anair conditioning system which can provide both heating and cooling forthe passenger compartment; and to a method of operating such an airconditioning system.

BACKGROUND OF THE INVENTION

Air conditioning systems for the passenger compartments of motorvehicles are well known. In general, these systems comprise an insideheat exchanger (located within the passenger compartment) and an outsideheat exchanger (located outside the passenger compartment). A pair offluid passages connect the heat exchangers to allow the circulation offluid through the heat exchangers. An expansion device is positioned inone of the fluid passages. A compressor and accumulator/dryer ispositioned in the other fluid passage. When fluid is pumped by thecompressor through the outside heat exchanger, the expansion device, theinside heat exchanger and the accumulator/dryer in succession, airpassing through the inside heat exchanger is cooled as the air flowsinto the passenger compartment. When fluid is pumped in the reversedirection through the inside heat exchanger, the expansion device, theoutside heat exchanger and the accumulator/dryer in succession, airpassing through the inside heat exchanger is heated as the air flowsinto the passenger compartment. A reversing valve can be positioned inthe other fluid passage to provide the required flow direction for thefluid.

During the cooling cycle, the air passing through the inside heatexchanger may be de-humidified, leading to a build-up of condensation onthe inside heat exchanger. During a subsequent heating cycle, thecondensation may be evaporated, increasing the risk of hot saturated airentering the passenger compartment. This potential problem may beavoided either by limiting the time of the heating cycle, or by using asecond inside heat exchanger during the heating cycle (with the firstinside heat exchanger only being used during a cooling cycle). Both ofthese potential solutions have limitations.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the above mentionedproblem.

An air conditioning system in accordance with the present invention fora passenger compartment of a motor vehicle comprises a first heatexchanger positionable outside the passenger compartment; a second heatexchanger positionable inside the passenger compartment; a first fluidpassage between the first and second heat exchangers; a second fluidpassage between the first and second heat exchangers; an expansiondevice positioned in the first fluid passage; an electronicallycontrolled variable stroke compressor for pumping fluid into the secondfluid passage and either in a first direction sequentially through thefirst heat exchanger, the expansion device, and the second heatexchanger, or in a second direction sequentially through the second heatexchanger, the expansion device, and the first heat exchanger; reverseflow means in the second fluid passage for controlling the direction ofthe flow of fluid; first sensing means providing a first output signalindicative of the actual temperature of the air leaving the second heatexchanger; second sensing means providing a second output signalindicative of the humidity of the air leaving the second heat exchanger;third sensing means providing a third output signal indicative of thetemperature of the air in the passenger compartment; and control meanselectrically connected to the first, second and third sensing means andto the compressor for receiving the first, second, and third outputsignals, for determining a maximum temperature for the air leaving thesecond heat exchanger dependent on the second and third output signals,for comparing the determined maximum temperature to the actualtemperature, and for controlling the stroke of the compressor tomaintain the actual temperature at or below the determined maximumtemperature during fluid flow in the second direction.

Because of the use of an electronically controlled variable strokecompressor, the present invention allows more precise control of thepumping capacity of the compressor when the air conditioning system isheating the passenger compartment. The present invention provides an airconditioning system which operates dependent on sensed conditions toreduce the risk of unacceptable levels of hot saturated air reaching thepassenger compartment without the need for limiting the time ofoperation of the heating cycle and without the need for another heatexchanger inside the passenger compartment.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic view of an air conditioning system in accordancewith the present invention;

FIG. 2 is a cross-sectional view of a compressor and control valve foruse in the air conditioning system of FIG. 1; and

FIG. 3 is a flow chart showing one example of control of the compressorof the air conditioning system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 of the drawings, the air conditioning system 10 inaccordance with the present invention is for use in a motor vehicle forheating or cooling the passenger compartment (not shown) of the motorvehicle. The air conditioning system 10 comprises the usual componentsof a compressor 12, an outside heat exchanger 14, an orifice tube orother expansion device 16 (such as a thermal expansion valve), an insideheat exchanger 18, and an accumulator/dryer 20. A first fluid passage 22fluidly connects the outside heat exchanger 14 with the inside heatexchanger 18 by way of the expansion device 16. A second fluid passage24 fluidly connects the outside heat exchanger 14 with the inside heatexchanger 18 by way of a reversing valve 32. The compressor 12 and theaccumulator/dryer 20 fluidly connect with the second fluid passage 24 byway of the reversing valve 32.

During normal (cooling) operation of the air conditioning system 10, thereversing valve 32 is set to allow fluid flow in the direction X suchthat air passing through the inside heat exchanger 18 is cooled so thatthe air conditioning system operates to cool the passenger compartment.

When initial, supplemental, or prolonged heating of the passengercompartment is required, the reversing valve 32 is actuated to reversethe flow of refrigerant fluid (in the direction Y) through the insideheat exchanger 18, the orifice tube 16 and the outside heat exchanger14. In this mode, the air conditioning system 10 acts like a heat pumpsuch that air passing through the inside heat exchanger 18 is heated sothat the air conditioning system operates to heat the passengercompartment.

The compressor 12 is an electronically variable compressor the operationof which is controlled by an electronic displacement control valve 26.An example of a suitable compressor 12 and control valve 26 is shown inFIG. 2. The compressor 12 shown in FIG. 2 is a wobble plate compressor.As an alternative, a swash plate compressor may be used.

The compressor 12 includes a pulley 80 which is connected to a rotatableshaft 82, and which is driven by a belt 84. A wobble plate 86 is mountedon the shaft 82. The wobble plate 86 is connected to one or more pistons88. A crankcase chamber 90 is positioned on one side of the pistons 88,with the wobble plate 86 positioned in the crankcase chamber. An outletchamber 92 and a inlet chamber 94 is positioned on the opposite side ofthe pistons. The inlet chamber 94 is fluidly connected to theaccumulator 20. The outlet chamber 92 is fluidly connected to thereversing valve 32. The other components of the air conditioning system10 are fluidly connected as shown in FIG. 1. Fluid flow through thechambers 90, 92, 94, and hence the fluid pressure in the chambers, iscontrolled by the control valve 26.

The control valve 26 has a first port 96 fluidly connected to the outletchamber 92; a second port 98 fluidly connected to, and acting as aninlet to, the crankcase chamber 90; a third port 100 fluidly connectedto, and acting as an outlet from, the crankcase chamber 90; and a fourthport 102 fluidly connected to the inlet chamber 94. The control valve 26is electrically connected by a line 28 to a control unit 30 which ispreferably a microprocessor or other computer control unit. The controlunit 30 is electrically connected by a line 34 (FIG. 1) to a temperaturesensor 36 which monitors the temperature of the air leaving the insideheat exchanger 18; by a line 38 to an air humidity sensor or air dewpoint sensor 40 which monitors the humidity of the air leaving theinside heat exchange; and by a line 45 to a temperature sensor 46monitoring the temperature inside the passenger compartment. The controlunit 30 may also be electrically connected by a line 50 to a manuallyoperated control device 52 located inside the passenger compartment andoperable by a passenger in the motor vehicle to select a requiredtemperature inside the passenger compartment.

The stroke of the compressor 12 (or, more precisely, the displacement orstroke of the pistons 88) is controlled by the operation of the controlvalve 26. The duty cycle of the control valve 26 is actuated to adjustcrankcase fluid pressure Pc in the crankcase chamber 90; the inletsuction fluid pressure Ps in the inlet chamber 94; and the dischargefluid pressure Po in the outlet chamber 92. When the crankcase fluidpressure Pc is substantially the same as the inlet suction fluidpressure Ps, the stroke of the compressor 12 is at a maximum. When thecrankcase fluid pressure Pc is greater than the inlet suction fluidpressure Ps, the stroke of the compressor 12 is reduced from the maximumstroke. By suitable control of the control valve 26, the stroke of thecompressor 12 can be controlled.

In an alternative arrangement, the stroke of the compressor 12 may becontrolled by an electronic control valve that meters fluid flow fromthe outlet chamber 92 to the crankcase chamber 90 and uses a fixed bleedfrom the crankcase chamber to the inlet chamber 94. In a furtheralternative, the reverse arrangement may be used—that is metering fluidflow from the crankcase chamber 90 to the inlet chamber 94 and using afixed bleed from the outlet chamber 92 to the crankcase chamber. As withthe duty cycle arrangement described above, these alternativearrangements also control the stroke of the compressor 12 by effectingthe pressure in the crankcase chamber 90 and the pressure balance acrossthe piston 88.

In accordance with the present invention, the control unit 30 monitorsthe signals from the sensors 36, 40, 46 and the control device 52 andcontrols the operation of the control valve 26, and hence the operationof the compressor 12 dependent on the sensed signals. Such anarrangement provides more precise control of the pumping capacity of thecompressor 12 during the heating cycle of the air conditioning system 10when the passenger compartment is being heated in order to substantiallyprevent an unacceptable build-up of hot saturated air reaching thepassenger compartment.

The control sequence performed by the control unit 30 for the operationof the compressor 12 during passenger compartment heating is shown inFIG. 3. The sequence begins with an initial request, step 54, forpassenger compartment heating. The control unit 30 actuates the controlvalve 26 to provide a minimum operating stroke for the compressor 12 atstep 56. The control unit 30 then checks that any delay criteria are metat step 58. If not, the control unit returns to step 56. If yes, thecontrol unit 30 proceeds to step 60 and actuates the control valve 26 toincrease the stroke of the compressor 12. Next, at step 62, the controlunit 30 monitors the temperature reading from the sensor 46 and at step64 compares this temperature reading to a predetermined maximum level.If the temperature reading from the sensor 46 is equal to or above thepredetermined maximum, the control unit 30 turns off the compressor 12(step 66). If the temperature reading from the sensor 46 is below thepredetermined maximum, the control unit 30, at step 68, monitors thehumidity reading from the sensor 40. Next, at step 70, the control unit30 determines a maximum temperature T_(MAX) based on the measuredreadings from the sensors 40 and 46. The value of T_(MAX) is apredetermined maximum value for the temperature of the air leaving theinside heat exchanger 18, at the measured readings from the sensors 40and 46, above which there is a significant risk of the generation of anunacceptable build-up of hot saturated air reaching the passengercompartment. The predetermined values for T_(MAX) may be determinedexperimentally or by calculation. At step 72, the control unit 30monitors the actual value of the temperature T_(ACC) of the air leavingthe second heat exchanger 18 from the sensor 36. Next, at step 74, thecontrol unit 30 compares the value of T_(ACC) measured at step 72 to thevalue of T_(MAX) determined at step 70. If T_(ACC) is above T_(MAX), thecontrol unit 30 actuates the control valve 26 to reduce the stroke ofthe compressor 12 at step 76. If T_(ACC) is below T_(MAX), the controlunit 30 actuates the control valve 26 to increase the stroke of thecompressor 12 at step 77. If T_(ACC) is equal to T_(MAX), the controlunit 30 leaves the control valve 26 unchanged to maintain the stroke ofthe compressor 12 at step 78. Following step 76, or step 77, or step 78,the control unit 30 returns to step 62 and repeats the subsequentsequence, or goes to step 66 if passenger compartment heating is nolonger required.

The present invention therefore provides closed-loop control of theoperation of the compressor 12 based on measured parameters. Bymaintaining the actual temperature T_(ACC) of the air leaving the insideheat exchanger 18 at or below the determined maximum temperature T_(MAX)for the measured humidity of the air leaving the inside heat exchangerand the passenger compartment temperature, the risk of unacceptablelevels of hot saturated air reaching the passenger compartment issignificantly reduced.

In an alternative arrangement, because the actual temperature T_(ACC) ofthe air leaving the inside heat exchanger 18 are dependent on fluidpressure, the discharge pressure from the compressor 12 may be measured,and the value used by the control unit 30 to calculate the value of theactual temperature, rather than use the sensor 36 to measure the actualtemperature. In this case, the discharge pressure may be compared to amaximum pressure which can be equated to the determined maximumtemperature T_(MAX) for controlling the operation of the compressor 12.

Alternative reverse flow means, rather than the reversing valve 32, maybe provided in the air conditioning system in accordance with thepresent invention. The control unit 30 may also be connected to thereversing valve 32 for controlling operation of the reversing valve.

What is claimed is:
 1. An air conditioning system for a passengercompartment of a motor vehicle comprises a first heat exchangerpositionable outside the passenger compartment; a second heat exchangerpositionable inside the passenger compartment; a first fluid passagebetween the first and second heat exchangers; a second fluid passagebetween the first and second heat exchangers; an expansion devicepositioned in the first fluid passage; an electronically controlledvariable stroke compressor for pumping fluid into the second fluidpassage and either in a first direction sequentially through the firstheat exchanger, the expansion device, and the second heat exchanger, orin a second direction sequentially through the second heat exchanger,the expansion device, and the first heat exchanger; reverse flow meansin the second fluid passage for controlling the direction of the flow offluid; first sensing means providing a first output signal indicative ofthe actual temperature of the air leaving the second heat exchanger;second sensing means providing a second output signal indicative of thehumidity of the air leaving the second heat exchanger; third sensingmeans providing a third output signal indicative of the temperature ofthe air in the passenger compartment; and control means electricallyconnected to the first, second and third sensing means and to thecompressor for receiving the first, second, and third output signals,for determining a maximum temperature for the air leaving the secondheat exchanger dependent on the second and third output signals, forcomparing the determined maximum temperature to the actual temperature,and for controlling the stroke of the compressor to maintain the actualtemperature at or below the determined maximum temperature during fluidflow in the second direction.
 2. An air conditioning system as claimedin claim 1, wherein the control means comprises a microprocessorelectrically connected to the first, second and third sensing means, anda control valve connected to the compressor and operated by themicroprocessor to control the stroke of the compressor.
 3. An airconditioning system as claimed in claim 1 or claim 2, wherein the firstsensing means is a temperature sensor which monitors the temperature ofthe air leaving the second heat exchanger.
 4. An air conditioning systemas claimed in claim 1, further comprising a manually operable controldevice which is electrically connected to, and providing an outputsignal to, the control means.
 5. An air conditioning system as claimedin claim 4, wherein the reverse flow means comprises a reversing valve.6. A method of operating an air conditioning system as claimed in claim5, comprising the steps of determining the temperature of the airleaving the second heat exchanger; measuring the humidity of the airleaving the second heat exchanger; measuring the temperature of the airin the passenger compartment; determining a maximum temperature for theair leaving the second heat exchanger based on the measured humidity andthe measured temperature of the air in the passenger compartment;comparing the determined maximum temperature for the air leaving thesecond heat exchanger with the determined temperature of the air leavingthe second heat exchanger; and controlling the stroke of the compressorto maintain the determined temperature at or below the determinedmaximum temperature.
 7. A method as claimed in claim 6, wherein themaximum temperature is determined from predetermined values of themaximum temperature which have been derived experimentally from measuredvalues for the humidity of the air leaving the second heat exchanger andthe temperature of the air in the passenger compartment.
 8. A method asclaimed in claim 7, wherein the step of determining the temperature ofthe air leaving the second heat exchanger comprising measuring thetemperature of the air leaving the second heat exchanger.